High yield pulp from hardwoods



Dec. 19, 1961 T. AITKEN ETAL 3,013,934

HIGH YIELD PULP FROM HARDWOOD-S Filed Aug. 12, 1958 CHIPS (HARDWOOD) DIGESTER BLOWPIT DEWATERING SCREEN SPENT CHELATING LIQUOR AGENT FIRST STAGE REFINER SECON D STAGE REF INER SCREEN CLEANERS TWO STAGES) ATTZRNEYS.

States Patent Ofiice Patented Dec. 19, 1961 Our present invention relates to the high yield pulping of hardwood, and to the pulp so produced.

It is well-known that because of the increasing scarcity of softwods, and the corresponding greater availability of hardwoods, much experimental work has been expended to improve hardwood pulps to the end that they can replace the softwood pulps to an increasing degree. In the production of newsprint and similar inexpensive publication papers, the common practice is to grind spruce logs, for example, to produce so-called groundwood with yields as high as 96%. When it is attempted to grind hardwoods in a similar manner, the pulps produced are generally unsatisfactory. One attempt to produce a high yield hardwood pulp, mainly by mechanical attrition, has been to do away with the conventional log grinding machines, and instead to chip the hardwood logs in conventional manner, and to subject the chips to the llberizing action of a disk refiner, with preferably a pre-treatment of the chips prior to fiben'zing. One such process has been to subject the chips to a cold caustic pro-treatment, followed by either fiberizing in disk refiners, or a combination of screw pressing and disk refining. The pulp produced however, lacks brightness to a considerable degree, and for use in white publication papers requires bleaching.

The so-called neutral sulfite semi-chemical process (NSSC) has been practiced for many years, in which the cooking agent is a solution of sodium sulfite (Na SO which is buttered by the presence of a small amount of sodium carbonate or sodium bicarbonate. In this process however, the chips are cooked to a much lower yield (not over 80%).

In contradistinction to the NSSC process, our invention is concerned with pulp of much higher yield and higher initial brightness, which is had by employing substantially neutral sodium sulfite solution to treat hardwood chips for a short period at low temperatures, followed by fiberizing in a disk refiner. Thus in one specific embodiment of our invention, we treat hardwood chips for a period not exceeding 30 minutes at temperatures not exceeding 140 C. fol-lowed by fiberizing the so treated chips in a refiner of the double disk type to give a yield in the range of 88 to 93% (oven dried pulp to oven dried wood). The pulps so obtained, given the proper hardwoods to start with, will have an initial brightness of from 60 to 75% on the General Electric scale, a high opacity-90% on the Bausch '& Lomb contrast ratio instrument, high bulk of 2.4 to 2.8 cc.s per gram at 100 seconds Williams slowness, and in general be of a quality comparable to that of softwood groundwood.

It is therefore the object of the present invention to pulp hardwoods of suitable nature, i.e., those which have intrinsically less coloring matter, to give a'pulp of the above characteristics, which i of a quality and strength comparable to that of softwood groundwood. The invention further involves specific conditions of hardwood treatment as set forth below, and the features of novelty will be pointed out in the appended claims.

The invention will be best understood by reference to the following detailed example illustrative thereof, reference being had to the accompanying flow sheet forming a partof this disclosure.

A mixture of hardwoods was used as the starting material, the composition of the mixture being as follows:

1 Percent Red maple 36 Sugar maple 20 7 Yellow poplar 20 Ash ll Beech 4 Yellow birch 4.5 Black birch 4.5

This mixture of Woods was chipped to the usual size, screened and -0 pounds thereof (oven dry basis-OD) was placed in the digester as indicated in the flow sheet. A cooking liquor (white liquor) was prepared of the following composition: Na SO -50 g./l.; NaOH3.4 g./l.; and sufiicient pounds thereof were added to the digester already containing the chips to give a liquor to chip ration of 5 including moisture in the' chips, making the amount of sodium sulfite used 22% of the OD chips, and that of the sodium hydroxide 1.5% of the OD chips. The liquor was charged to the digester at 150 C. The digester having been capped, suilicient steam was added to bring the temperature of the mixed liquor and chips to C., where-upon nitrogen gas under pressure was admitted to the top of the digester to bring the pressure therein to approximately 100 p.s.i.g.

The digester was held at this pressure and temperature for 20 minutes, after which it was blown. The chips were pumped to the dewatering screen and de-wtaered in the usual manner with the production of spent liquor, a portion of which was saved for reuse, since it still contained a considerable amount of unreacted sodium sulfite. The dewatered pulp was then diluted to a consistency of approximately 5% with water containing on the order of 0.14 g./l. sodium tripoly-phosphate. Other iron sequestering or chelating agents in suitable concentration may also be used. These agents are helpful in maintaining the brightness of the pulp, since the diluting water, which may be mill water, will unavoidably have picked up a minute amount of iron which otherwise would apprecia'bly drop the brightness of the pulp. The chelating agent may advantageously be added to the refiner during the fiberizing operation so that the pulp will be thus fortified when the mill water is added. The so treated pulp was then passed to the refiner of the doubledisk type, with the disks set fairly wide apart, e.g., .050 inch. After the first pass therethrough, the pulp was pumped to dewatering screens and refined again in the double disk refiner at -.001 inch setting. Then the pulp was washed, screened, and cleaned in a cleaner of the continuous centrifugal type, following which the pulp was subjected to the action of S0 for the purpose of preserving-its brightness. When the pulp is made into a wet lap, as is frequently the case at this point, the S0 in aqueous solution may be sprayed upon the lap. Otherwise the S0 may be mingled with the pulp in any suitable nianer. In practice, an amount of S0 equal to 1% of the OD pulp, has been found'satisfactory.

COMMENTS ON THE STEPS OF THE PROCESS Composition of hardwoods The following hardwood species yielded unbleached pulps of a brightness of 67 or higher; cottonwood, aspen, maple, birch, beech, ash, yellow poplar, serviceberry,

sycamore. Others, such as'red and black oak, hickory,

willow and gum, which yielded a brightness of slightly less than 67, may be used in minor proportions.

The following species gave a pulp which was still less of hardwood are delivered in a mixed state, in which case, special attention should be given to sorting of such wood before commencing the process.

Cooking conditions The cooking period should not be less than 5 minutes or more than 30 minutes, with preference given to the range of 20 minutes more or less. The permissible temperature range is from 110 to 140 C. with pressures ranging from 5 to 150 pounds per square inch gauge. An unusual property of the pulp produced by the instant process is that of becoming more opaque as the refining or beating time is increased, i.e., as the pulp freeness is reduced. When the period of cook exceeds 30 minutes, not only is the opacity less, but beating or refining no longer improves it. Also the yield goes below 88%. The use of nitrogen or other inert gas to create a higher pressure in the digester than the autogenous one, is of advantage in that it facilitates impregnation of the chips, whereby fewer chips are obtained in which the cores are uncooked. While it can be omitted, its use is favored since without it, greater power is required in the fiberizing step, and the brightness is diminished appreciably. In lieu of nitrogen or other inert gas, air can be used, but the use of air results in excessive oxidation of the sodium sulfite to sodium sulfate, which is inert. It is also possible to use steam at say 100 pounds pressure, by admitting it to the top of the digester where it will not cook the wood to an appreciably higher temperature. The equivalent hydraulic pressure may likewise be used.

While the amount of sodium sulfite may vary from 10 to 30% on the OD wood, an average figure of around 22% is preferred. Where oxidation is not permitted to occur during the digestion, it is found that only about 3 of the sodium sulfite is consumed, thus making it desirable to reuse the spent liquor after refoi'tifying it to original strength with new chemicals. The cooking should be regulated so as to give a yield in the approximate range of 88 to 93%. If a higher yield is sought, the wood is insufficiently treated with the cooking agent, and excessive fiberizing power is required. On the other hand, if the wood is cooked materially below the range given, the brightness and opacity are adversely affected.

An important ingredient in the cooking liquor is the small amount of alkali, i.e., from /2 to 3% NaOH or the equivalent, (e.g., Na CO NaHCO Na silicate, and others). Such additions make the pulp initially obtained, defiber more readily with marked saving of power, but without appreciably decreasing the pulp yield. Otherwise stated, a stronger pulp will be had for the same amount of power expended in the refiner. The effect of the alkali is shown in the following table:

Amount Strength of NaOH number Freeness G.E. B&L

added, Yield, at Canaafter brightopacity, percent percent dian std., cleaning ness, percent on OD ireeness Can. std. percent pulp of 200 The amount of refining was constant in the case of the above results. The amount of alkali may otherwise be gauged by measuring the pH at the end of the cook. Without the addition of alkali, this pH will vary from 7.0 to 7.3, whereas a final pH of 7.5 to 8.1 has been found most beneficial. The effect of the alkali is further shown by the fact that, if no alkali is used, about one third more power is required in the refiner to bring the freeness of the pulp to the same value as would obtain with the optimum amount of alkali added. The addition of alkali also increases the strength of the pulp up to twice that of the pulp prepared with Na SO alone. Addition of alkali in excess of the quantities shown results in a pulp deficient in brightness and may bring the yield below 88%. While the use of a double disk refiner is preferred, one of the single disk type may be used, if desired.

Preserving brightness The addition of the chelating agent, which may include various ones other than the sodium polyphosphate mentioned, e.g., the Versenes and sodium hexametaphosphate, operates as has been stated to nullify the adverse effect of iron that is present in solution. In the carrying out of the process, it is desirable to use stainless steel digesters and in general to take precautions to keep down the presence of iron to a minimum. Even after addition of the chelating agent, some loss of brightness will occur upon storage. This, however, may be minimized by the action of the S0 which effects a substantial restoration of the original brightness. The action of the S0 is indicated in the following table:

Brightness After refining 70.3 After screening 65.3 After cleaning 61.6 After scouring with S0 and standing 48 hours 70.4

The loss of brightness after cleaning was due to not using any chelating agent immediately following the refining. Had such chelating agent been used, the loss of brightness would have remained within a point or two of the original. However the use of the S0 is helpful in insuring the continued brightness of the pulp when stored for extended periods of time.

The pulp of the present invention thus has a brightness which enables it to be used without any bleaching step to make a blended sheet acceptable for various purposes. For example, a blended pulp consisting of of the pulp produced according to the present process, and 15% chemical pulp, gives a sheet of satisfactory newsprint quality. For light weight publication papers, i.e., for book and magazine purposes, 40% chemical pulp may be used.

When pulp yields are referred to, it will be understood that they are based upon the ratio of oven dried pulp (not air dried) to oven dried wood.

The pulp according to the present invention may be bleached like conventional groundwood to a higher brightness, if this is desired.

The improved process lends itself readily to being carried out in a continuous manner.

Various changes may be made in the steps specifically set forth without depanting from the spirit of our invention or the scope of the appended claims.

We claim:

1. In the method of pulping hardwood with a cooking liquor consisting essentially of sodium sulfite additioned by alkali, in which the cooked wood is fiberized in a rotary disk refiner, the steps of cooking the hardwood in the form of chips with said liquor in which the amount of said sodium sulfite present is from 1030% based on the OD wood and the amount of alkali present computed as NaOH is /23% based upon the OD wood, the temperature of the cook being maintained in the range from -l40 C., and the duration of the cook being not substantially in excess of 30 minutes, whereby the pH of the liquor at the end of the cook does not fall below approximately 7.5 and the pulp yield is from 8893%.

2. The method according to claim 1 in which pressure in excess of the autogenous is maintained upon the chips undergoing cooking in the range of 5 to pounds per square inch gauge.

3. The method according to claim 1 plus the added steps of diluting the pulp from the refiner with water and inhibiting the effect of iron in the added water by the action of an added chelating agent.

4. The method according to claim 3 plus the added step of adding S0 to restore the brightness of the pulp.

5. The method according to claim 3 plus the added steps of subjecting the pulp to a second fiberizing step in the revolving disk refiner, screening the pulp and adding S0 to the accepted pulp to restore brightness.

6. The method according to claim 3 in which the chelating agent is sodium tripolyphospate.

7. The method according to claim 1 in which the wood started with is red maple or other hardwood having at least the same intrinsic brightness as red maple.

8. A hardwood pulp produced according to the method of claim 7 composed of from 88-93% of the wood from which it was derived, having an original brightness of from 60-75% on the General Electric scale, and an opacity of approximately 90% when measured on the Bausch & Lomb contrast ratio instrument, and a bulk of said pulp further having the property of becoming more opaque upon additional refining or beating.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Rue, A. Semi-Chemical P-ulping Process from Chem.

2.4-2.8 cc.s per gram at 100 seconds Williams freeness, 15 & Met. Eng, October 19-27, pp. 6111, 612, 613. 

1. IN THE METHOD OF PULPING HARDWOOD WITH A COOKING LIQUOR CONSISTING ESSENTIALLY OF SODIUM SULFITE ADDITIONED BY ALKALI, IN WHICH THE COOKED WOOD IS FIBERIZED IN A ROTARY DISK REFINER, THE STEPS OF COOKING THE HARDWOOD IN THE FORM OF CHIPS WITH SAID LIQUOR IN WHICH THE AMOUNT OF SAID SODIUM SULFITE PRESENT IS FROM 10-30% BASED ON THE OD WOOD AND THE AMOUNT OF ALKALI PRESENT COMPUTED AS NAOH IS 1/2-3% BASED UPON THE OD WOOD, THE TEMPERATURE OF THE COOK BEING MAINTAINED IN THE RANGE FROM 110-140*C., AND THE DURATION OF THE COOK BEING NOT SUBSTANTIALLY IN EXCESS OF 30 MINUTES, WHEREBY THE PH OF THE LIQUOR AT THE END OF THE COOK DOES NOT FALL BELOW APPROXIMATELY 7.5 AND THE PULP YIELD IS FROM 88-93%. 